Childhood-onset Obsessive-compulsive disorder (OCD) and tic disorders appear to arise, in some cases, as sequelae of common childhood infections, including Group A beta-hemolytic streptococcal (GABHS) infections. Children whose symptoms begin or exacerbate following GABHS infections may belong to a subgroup of neuropsychiatric disorders identified by the acronym PANDAS (for Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcal infections). The subgroup shares several common clinical characteristics in common, and also may share a common pathophysiology for their symptoms. Investigations of the nature and treatment of this novel subgroup are ongoing. For example, data from a recently completed community study demonstrated that transient motor tics are common among elementary school students, with a monthly prevalence rate of 3% - 10%. Further, tics are more frequently present during months when streptococcal infections are increased (November through February) than during the spring months(March through June). During this reporting period, additional experience has been gained in the use of an immunomodulatory therapy, plasma exchange, for the treatment of children with acute, severe OCD and tics. Plasma exchange had been previously shown to be strikingly and significantly superior to placebo, with the average improvement exceeding 50% on some measures at one-month follow-up (Perlmutter et al., Lancet 1999;354:1153-1158.) Improvements were sustained at one year follow-up evaluations, as well. The mechanism of action of therapeutic plasma exchange is currently under investigation in the PANDAS cohort. A study is also underway to determine whether long-term antibiotic administration (prophylaxis) will be successful in preventing post-streptococcal symptom exacerbations. In addition, several studies of Sydenham's chorea (the neurological manifestation of rheumatic fever) are continuing to determine the nature of the post-streptococcal autoimmunity, as well as to identify more effective treatment strategies.